Frequency converter



J. F.V FARRxNGToN FREQUENCY CONVERTER Filed March 16, 1958 INVENTORATTORNEY July 18, 1939.

y F. FARRINGT 'N B Patented July 18, 1939 UNITED STATES PATENT OFFICEFREQUENCY CONVERTER Wale Application March '16, 1938, Serial No. 196,124

7 Claims.

This invention relates generally to frequency converters, andparticularly to frequency converters adapted to be utilized inmolulated-carrier signal receivers of the superheterodyne type.

In frequency converters of superheterodyne receivers, there is provideda local oscillator with a frequency-determining circuit for generatingoscillations which are modulated with a received signal to produce anintermediate-frequency signal. There is a tendency for the modulator toaffect the frequency of the local oscillations. This variation of theoscillator frequency is due to the fact that the modulator is usuallycoupled relatively closely to the frequency-determining circuit of theoscillator so that changes in the impedance of the modulator gridcircuit to which the oscillator is coupled affect the natural resonantfrequency of the frequency-determining circuit. Suoh changes inimpedance may be caused by variations in the supply potentials or A. V.C.

potential applied to the modulator. Various attempts have been made toeliminate this difllculty. One such arrangement utilizes a commoncathode resistor for coupling the oscillator section and the modulatorsection of a frequency converter.

With this arrangement the effect of variations in impedance ortrans-conductance of the modulator section of the frequency converterupon the requency of the oscillator is materially reduced. However, whenthis circuit is utilized in a superheterodyne receiver, the eflciency ofthe converter is reduced by degeneration due to the cathode resistorwhich decreases the signal amplitude applied to the input electrodes ofthe modulator and which also applies to the input electrodesintermediate-frequency Wavesin such phase as to reduce theintermediate-frequency output.

It is an object of the present invention, therefore, to provide afrequency changer suitable for 4 use in superheterodyne receivers inwhich the oscillation frequency is substantially independent of theimpedance and transconductance char-- acteristics of the modulator.

It is another object of the invention to provide l a frequency convertersuitable for use in superheterodyne receivers in which the modulator isnot substantially coupled with the frequency-determining circuit of theoscillator.

In accordance with preferred embodiments of -n the invention, there isprovided a frequency converter including an oscillator section and amodulator section, each section having a space current path and inputand output electrodes. A resistor is included in the space current pathof the oscillator section across which are developedy (CL o-20)oscillation voltages which are coupled to the modulator section in aconventional manner. In one embodiment. separate cathodes are providedfor the two space current paths, and the resistor is included in thecathode-ground circuit of the oscillator section. In any embodiment, theresistor or impedance across which the oscillation voltage is developedis not substantially coupled to the frequency-determining circuit of theoscillator so that changes in the operating potentials applied to themodulator, which cause variations in the impedance of the modulator gridcircuit to which the oscillator is coupled, do not appreciably affectthe frequency of the oscillator frequency-determining circuit,

Referring nowfto the drawing, Figs. 1 and 2 are circuit diagrams,partlyschematic, of complete superheterodyne receivers embodyingdifferent forms of the invention in the frequency converter or firstdetector of the receiver.

Referring now more particularly to Fig. 1, there is shown schematicallya complete superheterodyne radio receiver embodying the presentinvention in a preferred form. In general, the receiver includes a.radio-frequency amplifier Ill having its input circuit connected to anantenna I I and ground I2 and its output circuit connected to afrequency changer or oscillator-modulator I3. Connected in cascade withthe frequency changer I3, in the order named, are anintermediate-frequency amplifier Il of one or more stages, va detectorand automatic amplification control supply I5, an audio-frequency,Aamplifier I6 of one or more stages, and a sound reproducer I1. Anautomatic amplification control bias, derived from A. V. C. supply l5,is applied to the grids of one or more of the tubes of amplifier I0,oscillator-modulator I3, and one or more of the grids ofintermediate-frequency amplifier I4 in order to maintain the signalinput to detector I5 within a relatively narrow range for a wide rangeof received signal amplitudes. It will be understood that the variouscircuits just described, with the exception of frequency-convertercircuit I3, may be of a conventional construction and operation, thedetails of which are well known in the art, rendering detaileddescription thereof unnecessary. n

Considering briefly the operation of v the receiver as a whole andneglecting for the moment the operation of frequency converter I3, perse, presently to be described, a desired modulated signal is selectedand amplified by radio-frequency amplifier Il, converted to a modulatedintermediate-frequency signal in frequency chang- REISSUED serves tomaintain the volume output of the receiver within a relatively narrowrange for a wide range of received signal amplitudes. Y

Referring now more particularly `to the parts of the system involvingthe present invention, frequency converter I3 comprises a Amodulatorsection 20 and an oscillator section 2|. The modulator section 20comprises a mixer or modulator vacuum tube 22 having an input circuitcoupled to radio-frequency amplifier I0 and an output circuit coupled tointermediate-frequency amplifier |4. The oscillator section 2| comprisesa vacuum tube 23; a frequency-determining circuit connected to the gridcircuit of tube 23 and including a variable tuning condenser 24 and aninductance 25; and an oscillationfeed-backcircuit comprising inductance26 coupled to inductance 25 and connected to the anode of tube 23through a blocking condenser 21. The tube 23 includes an un-bypassedresistor 28 in its cathode-ground circuit, Resistor 28 and condenser 29serve to couple oscillator 2| to a second input or injector grid of tube22. Suitable operating potentials are provided for tubes 22 and 23 fromsources indicated on the drawing as -`C, -i-Sc. and +B.-

In considering the operation of the circuit just described, it will beseen that the oscillator section 2| is a conventional one, generatinglocal oscillations at the resonant frequency of thefrequency-determining circuit 24, 25, The variations in the platecurrent of tube 23 generate voltage waves of oscillator frequency acrossresistor 28 These waves are applied through condenser 28 to theinjectorgrid of modulator tube 22 and they modulate the received signals at thisfrequency. It will be seen that resistor 28 is not substantially coupledwith the oscillator frequency-determining circuit 24, 25. Furthermore,the value of resistor 28 can be made low as compared to the impedance ofthe injector grid circuit of tube 22, which is effectively in parallelwith it. Consequently, variations in the impedance of the injector gridcircuit, caused by changes in the operating potentials applied to themodulator tube 22, do not materially affect the frequency of theoscillator frequency-determining circuit 24, 25, or the amplitude of theoscillator voltage applied to the injector grid.

As illustrative of a specific applicationvutilizing the circuit of Fig.1, the following circuit constants are given:

Tube 22 'Iype 6L? Tube 23 Type 6C5 Resistor 28 ohms-.. 1,000 Resistor 28ohms 50,000

In Fig. 2 is shown a receiver embodying another form of the invention inwhich the space current pat-hs of the oscillator and modulator sectionsare included in a common envelope, comprising a to the output circuit ofradio-frequency amplier Il and the output circuit of the modulatorsection is coupled to the input circuit of intermediate-frequencyamplifier |4. 'Ihe oscillator circuit is similar to Fig. 1 except thatresistor 2l is omitted from the cathode circuit and replaced byresistors 45 and 46 in the space current path of the oscillator sectionof tube 4l. Resistor 4l and condenser 4l constitute the means forcoupling the oscillator to the injector grid of the modulator section ofvacuum tube 4l.

The operation of the circuit of Fig. 2 will be apparent from thedescription given above with respect to .the operation of the circuit ofFig. 1. It is seen that oscillation voltages are developed acrossresistor 45 due to variations in the space current of the modulatorsection. Resistor 4l is not substantially coupled to thefrequency-determining circuit due to the decoupling resistor 4l,Therefore, variations in the impedance of the modulator section ofvacuum tube 4I or the transconductance thereof do not materially aectthe resonant frequency of frequency-determining circuit 24, 25.

It will be understood that the coupling circuits, comprising elements23, 2l', and 2l or 4l, 4l', and 41, may be designed to have afrequencyresponse characteristic which varies over the operating rangeof the oscillator in a complementary manner to the voltage developedacross the output impedance in the space current path of the oscillator,thereby to maintain the voltage supplied to the modulator section fromthe oscillator section substantially constant.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modiflcations may bemadetherein without departing from the invention, and it is, therefore,aimed xn the appended claims to cover all such 'changes andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A frequency converter comprising a modulator section and anoscillator section, each of said sections including a space current pathcomprising input and output electrodes, a signalinput circuit coupled tothe modulator input electrode, a frequency-determining circuit coupledto said oscillator section, impedance means included in the spacecurrent path of said oscillator section and substantially uncoupled withsaid frequency-determining circuit, a second input electrode in thespace current path of said modulator section, and means for coupling anoscillation voltage developed across said impedance to said second inputelectrode to modulate the signal input to said modulator section,whereby the resonant frequency of said frequencydetermining circuit isnot appreciably affected by variations in the impedance of the circuitof said second electrode.

2. A frequency converter comprising a modulator section and anoscillator section, each of said sections including a space current pathcomprising input and output electrodes., a signalinput circuit coupledto the modulator input electrode, a frequency-determining circuitcoupled to said oscillator section, a resistor included in the spacecurrent path of said oscillator section and substantially uncoupled withsaid frequencydetermining circuit, a second input electrode in the spacecurrent path of said modulator section, and means for coupling anoscillation voltage developed across said resistor to said second inputelectrode to modulate the signal input to said modulator section,whereby the resonant frequency of said frequency-determining circuit isnot appreciably aifected by variations in the impedance of the circuitof said second electrode.

3. A frequency converter comprising a modulator section and anoscillator section, each of said sections including a space current pathcomprising input and output electrodes, a signalinput circuit coupled tothe modulator input electrode, a frequency-determining circuit includedin the input circuit of said -oscillator section, impedance meansincluded in the space current path of said oscillator section andsubstantially uncoupled with said frequency-determining circuit, asecond input electrode in the space current path of said modulatorsection, and means for coupling an oscillation voltage developed acrosssaid impedance to said second input electrode to modulate the signalinput to said modulator section,

whereby the frequency of said frequency-determining circuit is notappreciably affected by variations in the impedance of the circuit ofsaid second electrode.

4. A frequency converter comprising a modulator section and anoscillator section, each of saidsections including a space current pathcomprising input and output electrodes and separate cathodes, asignal-input circuit coupled to the modulator input electrode, afrequency-determining circuit coupled to said oscillator section, an

unbypassed resistor included in the cathode circuit of said oscillatorsection and substantially uncoupled with said frequency-determiningcircuit, a second input electrode in the space current path of saidmodulator section, and means for coupling an oscillation voltagedeveloped across said resistor to said second input electrode tomodulate the signal input to said modulator section, whereby theresonant frequency of said A frequency-determining circuit is notappreciably .in the space current path individual tosaid oscillatorsection and substantially uncoupled with said frequency-determiningcircuit. a second input electrode in the space current path of saidmodulator section, and means for coupling arl` oscillation voltagedeveloped across said impedance to said second input electrode tomodulate the signal input to said modulator section, whereby therresonant frequency (of said frequencyoscillator section, a rst resistorincluded in the 'lator section and an oscillatowction, each of said saidsections including a space current path comprising input and outputelectrodes, a signal-input circuit coupled to the modulatorinputelectrode, a tunable frequency-determining circuit coupled to saidoscillator section, a second input electrode in the space current pathof said modulator section, impedance means included in the space currentpath of said oscillator section and coupled to said second inputelectrode for coupling a voltage to said second electrode to modulatethe input signal to said modulator section, said impedance means beingsubstantially uncoupled with said frequency-determining circuit, wherebythe resonant frequency of said frequency-determining circuit is notsubstantially aected by variations inthe impedance of the circuit ofsaid second electrode, said impedance and-its coupling to said secondinput electrode being so proportioned that the voltage coupled to saidsecond input electrode from said impedance means is substantiallyconstant over the tuning range of said frequency-determining circuit.

JOHN F. FARRINGTON.

